PDE5 Inhibitors Class Overview Monograph

Mechanism of Action

PDE5 inhibitors block the catalytic site of phosphodiesterase type 5, the enzyme responsible for hydrolyzing cyclic guanosine monophosphate (cGMP) to inactive 5-GMP. Elevated intracellular cGMP activates protein kinase G, which dephosphorylates myosin light chain and opens potassium channels, reducing cytosolic calcium. The net result is smooth-muscle relaxation and vasodilation.

The NO-cGMP Pathway

Sexual stimulation triggers nitric oxide (NO) release from penile endothelium and non-adrenergic, non-cholinergic (NANC) neurons. NO activates soluble guanylate cyclase, generating cGMP. Without sexual stimulation, intracellular cGMP concentrations remain too low for PDE5 inhibitors to produce erection, a pharmacologically important point that distinguishes this class from pro-erectile agents that act independently of NO signaling. [1]

Selectivity Profile

PDE5 is expressed most densely in corpus cavernosum, pulmonary vascular smooth muscle, and prostatic stroma. All four approved agents show meaningful selectivity for PDE5 over PDE1 (cardiac and brain) and PDE3 (cardiac), limiting direct cardiac inotropy. Sildenafil and vardenafil also inhibit PDE6 in retinal photoreceptors at therapeutic doses, which accounts for the transient color-vision disturbance some patients report. Tadalafil inhibits PDE11 (expressed in skeletal muscle and testis) at concentrations reached with the 20 mg PAH dose; clinical significance of PDE11 inhibition remains under investigation. [2]

Why Nitrates Are Absolutely Contraindicated

Nitrates are NO donors. They raise cGMP by the same upstream pathway that PDE5 inhibitors protect downstream. Co-administration produces additive, unpredictable, and potentially fatal hypotension. The FDA label for every agent in this class carries a black-box-equivalent warning against any nitrate form, sublingual, transdermal, oral, or intravenous. [3]

Approved Agents: Pharmacokinetic Comparison

Four PDE5 inhibitors hold FDA approval in the United States. Their pharmacokinetic profiles differ enough that prescribing decisions should account for onset, duration, food effects, and elimination pathway.

Sildenafil (Viagra, Revatio)

Sildenafil was approved for erectile dysfunction in 1998 and for pulmonary arterial hypertension (as Revatio) in 2005. Oral bioavailability averages 41% with high first-pass metabolism via CYP3A4 (primary) and CYP2C9 (minor). Peak plasma concentration occurs at 30 to 120 minutes; a high-fat meal delays T-max by approximately 60 minutes and reduces C-max by 29%. [4] Half-life is 3 to 5 hours.

ED dosing: 50 mg as needed, titrated to 25 to 100 mg based on efficacy and tolerability. PAH dosing (Revatio): 20 mg three times daily. The SUPER-1 trial established the 20 mg three-times-daily regimen as effective in WHO functional class II, III PAH patients, with a 45-meter improvement in 6-minute walk distance vs. Placebo over 12 weeks (P<0.001). [5]

Tadalafil (Cialis, Adcirca, Alyq)

Tadalafil's 17.5-hour half-life permits once-daily dosing at 2.5 to 5 mg for ED and BPH, or as-needed dosing at 10 to 20 mg for ED. Bioavailability exceeds 80%. Food does not affect absorption. CYP3A4 is the primary metabolic route; inactive catechol glucuronide is renally excreted. [6]

The MTOPS trial (N=3,047) confirmed that alpha-1-adrenergic blockers and PDE5 inhibitors share overlapping mechanisms in prostatic smooth muscle, justifying the tadalafil 5 mg once-daily BPH indication. [7] Tadalafil 40 mg once daily (Adcirca) is approved for PAH; the PHIRST trial showed a 33-meter improvement in 6-minute walk distance vs. Placebo at 16 weeks (P=0.0004). [8]

Vardenafil (Levitra, Staxyn)

Vardenafil's oral bioavailability is 15% (extensive first-pass via CYP3A4 and CYP3A5). Onset is 25 to 60 minutes; half-life is 4 to 5 hours. High-fat meals reduce C-max by 18 to 50%. The orodispersible formulation (Staxyn 10 mg) bypasses some first-pass effect and has slightly higher bioavailability than the conventional tablet. Vardenafil prolongs the QTc interval in a concentration-dependent manner, prescribers should avoid it in patients with congenital long-QT syndrome or those taking Class IA or III antiarrhythmics. [9]

Avanafil (Stendra)

Avanafil received FDA approval in 2012. It is the most PDE5-selective agent currently approved, with roughly 100-fold selectivity over PDE6 compared to sildenafil's approximately 10-fold selectivity, translating to a lower incidence of visual side effects. [10] Onset may occur as early as 15 minutes in some patients. Half-life is 5 hours. Doses: 50, 100, or 200 mg as needed. CYP3A4 is the primary metabolic pathway.

Indications and Guideline Recommendations

Erectile Dysfunction

The American Urological Association (AUA) 2018 Erectile Dysfunction Guideline (updated 2024) classifies oral PDE5 inhibitors as first-line therapy for ED across all etiologies, including vasculogenic, psychogenic, and mixed-etiology disease. [11] The guideline states: "Oral PDE5 inhibitors are recommended as first-line therapy for erectile dysfunction in men without contraindications to their use."

A 2014 Cochrane systematic review (35 RCTs, N=6,659) found sildenafil produced a relative risk of successful intercourse of 3.52 (95% CI 2.90 to 4.26) vs. Placebo. [12] Number needed to treat for one additional successful intercourse attempt was approximately 1.7, among the highest treatment effects in outpatient urology.

Pulmonary Arterial Hypertension

Sildenafil (Revatio) and tadalafil (Adcirca/Alyq) carry FDA approval for WHO Group 1 PAH. The American College of Chest Physicians (ACCP) and the European Society of Cardiology (ESC)/European Respiratory Society (ERS) 2022 guidelines recommend PDE5 inhibitors as initial oral monotherapy for functional class II, III patients. [13] Combination therapy with an endothelin receptor antagonist (ERA) plus a PDE5 inhibitor is now the preferred approach based on the AMBITION trial (N=500), which showed combination ambrisentan-tadalafil reduced the risk of clinical failure events by 50% vs. Pooled monotherapy (P<0.001). [14]

Benign Prostatic Hyperplasia

Tadalafil 5 mg daily is the only PDE5 inhibitor with an FDA-approved BPH indication (approved 2011). The drug improves International Prostate Symptom Score (IPSS) by a mean of 5 to 6 points vs. 2 to 3 points with placebo, comparable to the improvement seen with alpha-blockers in short-term trials. [15] Combination tadalafil plus tamsulosin produces additive IPSS improvement but requires blood pressure monitoring given the shared hypotensive mechanism.

Dosing Reference Table

| Agent | ED (as-needed) | ED (daily) | PAH | BPH | |---|---|---|---|---| | Sildenafil | 25 to 100 mg 30 to 60 min before | Not approved | 20 mg TID | Not approved | | Tadalafil | 10 to 20 mg 30 min before | 2.5 to 5 mg/day | 40 mg/day | 5 mg/day | | Vardenafil | 10 mg (range 5 to 20 mg) | Not approved | Not approved | Not approved | | Avanafil | 50 to 200 mg 15 to 30 min before | Not approved | Not approved | Not approved |

Contraindications and Precautions

Absolute Contraindications

Concurrent use of any organic nitrate (nitroglycerin, isosorbide mononitrate, isosorbide dinitrate, amyl nitrite, nicorandil) is absolutely contraindicated. The FDA labeling for sildenafil requires a minimum 24-hour washout after the last PDE5 inhibitor dose before nitrates can be safely given; for tadalafil, the required washout extends to 48 hours given its longer half-life. [16]

Hypersensitivity to the specific agent or excipients is the second absolute contraindication. Vardenafil is contraindicated with Class IA (quinidine, procainamide) or Class III (amiodarone, sotalol) antiarrhythmics.

Relative Contraindications and Cautions

Use caution in patients with:

• Active unstable angina or recent MI (<90 days): insufficient safety data from controlled trials.
• Resting hypotension (systolic <90 mmHg) or resting hypertension (>170/110 mmHg).
• Left ventricular outflow obstruction (HOCM, severe aortic stenosis).
• Non-arteritic anterior ischemic optic neuropathy (NAION): post-marketing case reports associate PDE5 inhibitors with NAION recurrence; the 2024 AUA guideline recommends counseling patients with prior NAION before prescribing. [11]
• Hereditary degenerative retinal diseases (retinitis pigmentosa): theoretical concern given PDE6 inhibition.

Alpha-Blocker Interaction

Alpha-1-adrenergic blockers (tamsulosin, doxazosin, terazosin) and PDE5 inhibitors both lower blood pressure via smooth-muscle relaxation in different vascular beds. Co-administration produces additive hypotension, most pronounced with non-uroselective agents (doxazosin, terazosin). The FDA recommends initiating the PDE5 inhibitor at the lowest available dose when alpha-blockers are already prescribed and ensuring the patient is hemodynamically stable on the alpha-blocker first. Tamsulosin 0.4 mg is considered the safest alpha-blocker for co-administration with tadalafil 20 mg. [17]

Drug Interactions

CYP3A4 Inhibitors

Strong CYP3A4 inhibitors (ritonavir, ketoconazole, itraconazole, clarithromycin) increase PDE5 inhibitor plasma exposure substantially. Ritonavir increases sildenafil AUC by 11-fold; the FDA label for sildenafil (Viagra) limits the dose to 25 mg every 48 hours with ritonavir. Tadalafil AUC increases approximately 2-fold with ketoconazole 200 mg daily; dose should not exceed 10 mg every 72 hours. [18]

CYP3A4 Inducers

Rifampin reduces sildenafil AUC by approximately 90% through CYP3A4 induction. Patients on rifampin for tuberculosis are unlikely to achieve therapeutic sildenafil concentrations at standard doses. Similar reductions affect tadalafil and avanafil. Switching to a higher dose may not reliably overcome the induction, and the interaction should be documented as a clinical barrier. [19]

Antihypertensives

PDE5 inhibitors produce a mean systolic blood pressure decrease of 5 to 8 mmHg when taken alone. Co-administration with antihypertensives (amlodipine, enalapril, metoprolol) produced an additional 3 to 6 mmHg decrease in controlled pharmacokinetic studies but rarely caused symptomatic hypotension in otherwise healthy participants. Blood pressure monitoring is reasonable at the start of therapy in patients on multiple antihypertensives. [20]

Adverse Effects

Common Adverse Effects (Class Effect)

Headache and flushing are the most frequently reported adverse effects, occurring in 10 to 15% of patients at therapeutic doses, primarily due to systemic vasodilation. Nasal congestion occurs in approximately 4 to 9%. Dyspepsia is more common with tadalafil (up to 10%) than sildenafil, likely related to PDE5 inhibition in lower esophageal sphincter smooth muscle. Myalgia and back pain occur in up to 6 to 9% of patients taking tadalafil, attributed to PDE11 inhibition. [21]

Visual Disturbances

Transient color discrimination changes (blue-green hue, increased light sensitivity) occur in approximately 3% of patients taking sildenafil 100 mg and are significantly less common with avanafil. The mechanism is PDE6 inhibition in retinal cones. Effects resolve within hours of dosing and are not associated with permanent vision loss in most cases. [4]

NAION

Post-marketing reports link PDE5 inhibitor use to NAION, a form of optic nerve ischemia causing sudden monocular vision loss. The FDA added a labeling update in 2007 recommending that patients with risk factors for NAION (prior NAION, crowded optic disc, age >50, hypertension, diabetes, hypercholesterolemia, coronary artery disease, smoking) be counseled about this risk. Absolute incidence remains rare and causality has not been definitively established in controlled studies. [16]

Priapism

Priapism (erection lasting >4 hours) is an uncommon but urologic emergency. Risk is elevated in patients with sickle cell disease, leukemia, or multiple myeloma. Patients should be instructed to seek immediate care if an erection persists beyond 4 hours. [11]

Special Populations

Renal Impairment

Sildenafil: For patients with CrCl <30 mL/min or on hemodialysis, start at 25 mg. Tadalafil (ED): CrCl 31 to 50 mL/min, maximum 10 mg every 48 hours; CrCl <30 mL/min, maximum 5 mg. Tadalafil (PAH): avoid with severe renal impairment; no controlled data. Avanafil: no dose adjustment required for mild-to-moderate CKD; data are limited for CrCl <30. [6]

Hepatic Impairment

Sildenafil: Child-Pugh A, B, start at 25 mg. Child-Pugh C: not recommended. Tadalafil: Child-Pugh A, B, maximum 10 mg; Child-Pugh C, not recommended. Vardenafil: Child-Pugh B, start at 5 mg, maximum 10 mg; Child-Pugh C, not recommended. [9]

Older Adults

Clearance of sildenafil, vardenafil, and tadalafil is reduced in patients over 65, raising plasma concentrations by 40 to 90% relative to younger adults. Starting at the lowest available dose is appropriate. Polypharmacy risk (alpha-blockers, antihypertensives, nitrates for angina) is higher in this population and requires careful medication reconciliation before prescribing. [21]

Cardiovascular Risk Assessment Before Prescribing

The Princeton Consensus III guidelines (2012) stratify patients into low, intermediate, and high cardiovascular risk before PDE5 inhibitor prescribing. Low-risk patients (controlled hypertension, <3 CAD risk factors, NYHA Class I) may start therapy without further evaluation. Intermediate-risk patients require stress testing or cardiology referral. High-risk patients (unstable angina, uncontrolled hypertension, recent MI, high-risk arrhythmia) should defer PDE5 inhibitor use until cardiac status is optimized. [22]

Comparative Effectiveness

Head-to-head trial data directly comparing all four agents are limited. A 2013 network meta-analysis (58 RCTs, N=17,696) published in the European Urology compared sildenafil, tadalafil, vardenafil, and avanafil for ED and found no statistically significant difference in the primary outcome (International Index of Erectile Function [IIEF] domain 2 score improvement) between agents at recommended doses. [23] Agent selection therefore depends on patient-specific factors rather than efficacy hierarchy.

The practical framework HealthRX clinicians apply when selecting an agent:

1. Coital frequency >2 times/week: Once-daily tadalafil 5 mg avoids the need to plan around dosing windows and maintains steady-state plasma levels.
2. Spontaneous, infrequent intercourse: Avanafil 100 to 200 mg provides the fastest reliable onset (15 to 30 minutes) with less PDE6-related visual disturbance than sildenafil.
3. Concurrent BPH/LUTS: Tadalafil 5 mg daily addresses both conditions with one agent.
4. PAH requiring oral monotherapy: Either sildenafil 20 mg three times daily or tadalafil 40 mg once daily; tadalafil's once-daily regimen may improve adherence.
5. CYP3A4 inhibitor co-prescription (e.g., ritonavir): Tadalafil 5 mg every 72 hours produces the most manageable dosing schedule given the interaction.
6. History of visual symptoms on sildenafil: Switch to avanafil given superior PDE6 selectivity.

Monitoring and Follow-Up

Blood pressure measurement before and after initiating therapy is reasonable, particularly in patients on antihypertensives or alpha-blockers. Patients beginning tadalafil 5 mg once daily for BPH or daily ED therapy should have blood pressure checked within 4 weeks of initiation.

Medication reconciliation at every visit should confirm no new nitrate prescription. Patients admitted to emergency departments or cardiology wards often receive sublingual nitroglycerin without immediate knowledge of outpatient PDE5 inhibitor use; carrying a medication list or wearing a medical-alert identifier is prudent for patients with concurrent CAD risk.

Efficacy follow-up at 4 to 6 weeks allows dose titration. Patients failing 50 mg sildenafil without adequate efficacy should be counseled on adherence to administration instructions (empty stomach, adequate sexual stimulation, sufficient time post-dose) before escalating dose. Approximately 30% of initial PDE5 inhibitor "failures" respond to repeat dosing with corrected instructions. [24]